Since the alphabet is often used for numbering (especially in lists), I always imagined it extended as an infinite number line, like numerals. Following the same principle as numbers, when all of the single digit characters are used up, then you move to a second digit, and the "ones" place repeats over. With the letters, since there is no "zero", and also to have an even numbered base, this system is modified from regular decimal-style notation. Instead of the base being "A0"; the first two digit integer, it is the last single digit integer, "Z". The next integer after that is "AA". If this was done with numerals, the number line would be [no zero] 1, 2, 3, 4, 5, 6, 7, 8, 9, 0(ten), 11, 12, 13, 14, 15, 16, 17, 18, 19, 10, 21, 22, 23…94, 95, 96, 97, 98, 99, 90(one hundred), 01(one hundred one), 02, 03, 04, 05, 06, 07, 08, 09, 00, 111, 112, 113…
So with the letters, it is basically a modified base 26, or icosaheximal system!

Most alphabetical numbering systems that have gone above "Z", often for some reason use only double digits: AA, BB, CC, DD…ZZ, AAA, BBB, etc. The most common place I have seen this is on court exhibits (which in supreme or civil courts are actually usually bound copies of documents). One 1991 Manhattan case went all the way up to "eeeee"! (which would have the numerical value of 108). But this wastes digits. In that system, the number of digits is the multiple of 26, with the numerical value of the digits being what is added to the multiple of 26. So EEEEE in this system would simply be DE! Spreadsheet programs like the one in Microsoft Works use this system, for the columns (while the rows are regular numbers), and since the entire grid has 256 by 256 fields, the rows run from 0 to 255; while the columns run from A to IV!

Just like decimal places are figured by adding powers of ten (the ones place being 1 (100)× the numerical value of the digit, + the tens place at 10¹ × the numerical value of the digit, + the hundreds place at 10² × the numerical value, and so on. So in this case, the "a’s" place is 1 (260) × the numerical value of the digit, and the AA’s place is 26 × its numerical value, and the AAA’s is 26² × its numerical value. AA is 26×1 (the value of A) + 1=27. BB is 2×26=54 + 2 = 56. AAA is 26²=676 + 26 +1= 703. Now, how about a bigger one? My name, ERIC is 5×26³ + 18×26² + 9×26, + 3 (87880+12168+234+3)=100285.

Here is the addition and multiplication table for all of the letters from A to Z: